Description (adapted from the application): Extending a series of observations regarding the hypothesis that CFS patients can be stratified into putative neurologic and cardiologic groups, this project intends to use exercise and hypoxia as a physiologic stressors to perturb acute cognitive function. The underlying hypothesis is that subgroups of these patients have cognitive dysfunction that is not clinically apparent in a basal state. The investigators further hypothesize that in the face of physiologic stresses of maximal exercise, submaximal exercise and mild hypoxia mimicking 12,000 feet altitude, cognitive changes will become apparent. Subjects will undergo: ANAM Cognitive Test battery- following asymptotic training. Submaximal exercise tests to assess gas-exchange threshold; then sustained workload to 70 percent of the gas-exchange threshold. Ambulatory Activity Monitoring: 5 days prior and 14 days following exercise. Hypoxic challenge at 85 percent measured oxygen saturation. With the submaximal exercise challenge and the hypoxic challenge, subjects will have a comprehensive set of non-invasive physiologic investigations including photoplethsymography, impedance cardiography, and occlusion venous plesthymography. They will sequentially undergo the ANAM cognitive test battery in a pattern that includes intra-subject controls for order effects and training effects. Data will be analyzed with the group stratified by the operational definitions the putative groups noted in the core discussion.